Opto-mechanical devices are precision instruments useful for controlling the path along which light travels. This path is subject to six independently controlled constraints. A beam of light may travel on a particular path defined by specifying X, Y and Z coordinates. This beam of light is further defined in terms of its rotation about these same axes. This rotation about the X, Y and Z axes is referred to as roll, pitch and yaw respectively. The ability to accurately detect these parameters with a high degree of resolution is required for a variety of applications. For example, detection of a particular species of organic compound, e.g. those having dextrorotatory or levorotatory characteristics requires the ability to measure the rotation of light about an axis as it passes through a solution containing the compound being investigated. This same inventor, in U.S. Pat. No. 5,822,067, Oct. 13, 1998, the contents of which are herein incorporated by reference, describes an optical detection system for use in measuring optical activity within a sample as it flows through a detector consisting of a laser diode with a beam shaping means, a polarizing prism, a flow cell, and a means for attenuating laser fluctuation effects from the sensed signal. In order to control the rotation of the polarizing prism with a high degree of precision, it is known to utilize micro-precision rotation stages.
Micro-precision rotation stages suitable for such applications are available from a variety of manufacturers. Among these are the Melles Griot company, which markets a variety of stages having multiple drive knobs for vernier scale adjustment and operation at resolutions from 5 arc min to 18 arc sec. Another type of rotation stage is available from the Newport Corporation, which manufactures a broad variety of both manual and motorized rotary stages. The Newport devices provide rotary stages having a center aperture which permits the mounting and adjustment of rotating components in light transmissive applications. The Newport rotary stages provide preloaded bearings which are placed in precision-ground races. These stages provide resolution in the range of 4 arc sec-30 arc min.
The Aerotech Corp. manufactures a rotary stage with a clear aperture, marketed under the name ARS301, which utilizes a "sub-arc" second resolution drive mechanism, capable of achieving a 0.1 arc sec resolution. The rotating portion is supported by ball bearings and rotation is effected by manipulation of a steel spring which is placed circumferentially about the rotating portion and is extended or retracted via a finely-threaded adjustment screw.
The problem with these prior art devices is that the use of ball bearings fails to provide a system which accurately and repeatably translates the minute angular excursions of the rotary stage into true linear movement. The bearings are not perfectly round, and they are not being utilized in a manner consistent with their intended function. If a bearing and race combination is included, e.g in a wheel and axle assembly, the bearings make many complete revolutions per minute as they reduce the frictional forces which would otherwise hamper the wheel's rotation. In such an environment, minute inconsistencies in the bearings are of no real consequence. In environments such as that of the instant application, the bearings do not revolve, rather they are moved back and forth over a very small portion of their circumference, on the order of 5-10 degrees. In this type of an environment, inconsistencies in the "roundness" of the bearings result in a substantial reduction in the repeatable resolution which can be attained with such devices.
As an alternative to the use of ball bearings in limited movement applications, another type of flexural joint is available from the C-Flex Bearing Co. Both the C-Flex joint and a similar device available from the Lucas Aerospace Power Transmission Corp. are low hysteresis, frictionless joints designed to provide accurate linear movement in limited rotation environments. The design of these joints, do not, however, provide a clear central aperture, and therefore would not be useful for mounting of light transmissive devices such as polarizing prisms.
Thus, what is lacking in the art, is a rotary positioning device having a clear aperture for inclusion of light transmissive optical equipment which is capable of positioning such equipment for precise and reproducible rotation at resolutions finer than 0.1 arc seconds.